This application is related to Japanese applications Nos. 2001-194740 and 2002-054336, filed on Jun. 27, 2001 and Feb. 28, 2002 whose priorities are claimed under 35 USC xc2xa7 119, the disclosures of which are incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to a method for forming a plating film and to an electronic component having a plating film formed thereon by the same method. More particularly, the present invention relates to a method for forming a plating film which is lead-free and excellent in solderability and to an electronic component having a plating film formed thereon by the same method.
2. Description of Related Art
In the field of the electric and electronic industries, where soldering is generally used for formation of circuits, an Snxe2x80x94Pb containing alloy is typically used as a soldering material and for rapid and ensured soldering, a plating film of the SnPb-containing alloy is applied onto an object to be soldered before hand.
For example, as shown in FIG. 7, a photo coupled semiconductor device 40, which utilizes soldering, is constituted as follows. A light emitting element 42 and a photoreceptor 43 are connected to headers of lead frames 41, respectively, by die bonding. The lead frames 41 are made of a metal such as a Cu alloy or Fe alloy and are bent in advance. The light emitting element 42 and the photoreceptor 43 are connected to the adjacent lead frames 41, respectively, by wire bonding using gold wires 48. The light emitting element 42 is precoated with resin 44. The lead frames 41 having the light emitting element 42 and the photoreceptor 43 connected thereto by die bonding are positioned and opposed to each other by spot welding or using a loading frame set, and an inner package is formed by primary transfer molding using a light transmitting epoxy resin 45. Further, an outer package is formed by secondary transfer molding using a light shielding epoxy resin 46, and a non-glossy (non-shiny or lusterless) SnPb plating film is applied onto portions of the lead frames 41 extending outside the outer package.
Thus, SnPb, which is inexpensive, has a low melting point, and is excellent in processibility, has been used as a material for soldering and also as a material for a plating film. In recent years, however, it becomes urgent to replace SnPb by a material which does not contain lead (lead-free material), as an environmental protection measure. Examples of such a material for soldering include an alloy containing tin as a principal component together with one or more selected from silver, bismuth, copper, zinc and the like, and examples of such a material for a plating film include an alloy containing tin as a principal component together with one or more selected from silver, bismuth, copper, zinc and the like; gold; silver; palladium; and the like.
Japanese Unexamined Patent Publication No. 2000-26994 proposes an SnCu plating coat layer whose cryatal on a surface is fined so that the surface is in a gloss state or a half gloss state.
However, it has been found that a plating coat layer as mentioned above has a zero cross time of 5 seconds or more, indicating that the coat layer is poor in solderability and appearance when the solderability of the plating coat layer is evaluated by a meniscograph method by immersing a component which has the coat layer as mentioned above into a soldering bath filled with a soldering alloy comprising Snxe2x80x94Ag (3.5%) at 270xc2x0 C., at a predetermined rate and to a predetermined depth for five minutes and then picking it upward.
The present invention provides a method for forming a plating film, comprising the steps of: applying a plating film onto an object to be plated at a first density for a predetermined period in a plating bath having a cathode and an anode and capable of varying current and maintaining the object to be plated at a second current density lower than the first current density or immersing the object to be plated into a solution containing 5 to 10% wt of trisodium phosphate.
Further, the present invention provides an electronic component having a plating film formed thereon by the above method.
According to the present invention, it is possible to improve solderability of a plating film for conventional lead-free solder by a simple method, which allows the productivity to further enhanced, resulting in a plating film with reduced production costs.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.